1. Field of the Invention
The present invention relates to a sheet post-processing apparatus having a function of punching a sheet.
2. Description of the Related Art
Conventionally, some sheet post-processing apparatuses connected to an image forming apparatus are equipped with a sheet punching apparatus that punches a recording sheet. Sheet punching apparatuses include those of a press-punch type that temporarily stops sheets with images formed thereon when they are being conveyed and punches the sheets one by one, and those of a rotary type that punches sheets without stopping them. In general, hole positions are more accurate with the press-punch type than with the rotary type. The press-punch type, however, stops sheets one by one whenever it punches them, and hence it takes much time to punch the sheets.
To address this problem, according to Japanese Laid-Open Patent Publication (Kokai) No. 2000-334694, while sheets are being conveyed so as to be guided into a sheet punching apparatus, starting of a punch motor for punching is advanced so as to shorten the time from stop of sheet conveyance to start of punching so that high speed punching can be realized.
A description will now be given of a conventional sheet punching apparatus with reference to FIGS. 13 to 16.
FIG. 13A is a perspective view showing the conventional sheet punching apparatus, FIGS. 13B and 13C are views taken from directions F1 and F2, respectively, in FIG. 13A.
Referring to FIG. 13A, the conventional sheet punching apparatus causes a slider, which has cam grooves, to reciprocate by a punch motor 221 which is a drive unit, thus punching a sheet using punches integrated with pins moving inside the cam grooves. The pins engaging with the cam grooves cause the punches to reciprocate in a direction perpendicular to a direction in which the slider reciprocates.
FIGS. 14A and 14B are timing charts showing sheet conveyance, punch motor, and punch position in a punching process.
Referring to FIG. 14A, a time period Δt elapses from when a sheet stops and the punch motor 221 is started to when the punches actually punch the sheet. This time period Δt is set as an advance time period, and as shown in FIG. 14B, starting of the punch motor 221 and resumption of sheet conveyance are advanced by the time period Δt, so that the time period required for punching can be shortened.
In a case where, however, a sheet to be punched is a thick sheet, the load put on the punches when they punches the sheet is heavier compared to a plain sheet. For this reason, the punch motor 221 driving the slider lows down to a large degree.
FIG. 15A is a conceptual diagram showing positions of the punch over time with comparison between a thick sheet and a plain sheet. FIG. 15B is a diagram showing changes in positions of the punch and the cam groove in the slider.
Movement of the slider causes the pin to move along the cam groove in the slider. Stopping the punch motor 221 causes the slider to stop, but the punch motor 221 does not immediately stop moving and moves a small amount after being braked. As a result, the slider overruns.
As described above, the slider slows down when punching a thick sheet, and hence even when an attempt to stop the slider with the same timing as in the case of a plain sheet, the slider stops earlier compared to the plain sheet. For example, assume that when a plain sheet is to be punched immediately after a thick sheet is punched, punching is started with a predetermined advance time irrespective of a sheet type, in particular, a sheet thickness as in the prior art. Then, as shown in FIG. 15B, a position at which the punch stops in the case of the thick sheet is relatively close to a punching position compared to a position at which the punch stops in the case of the plain sheet.
Punching of a next sheet is carried out by reversing a direction in which the slider moves, and as shown in FIG. 15B, a position (L1) close to a punching position is a relative initial position of the slider and the punch. Assume that under the circumstances, the punch motor 221 is started so as to punch a next sheet with an advance time period set at Δt as shown in a timing chart of FIG. 16. The time it takes for the punch to actually punch the sheet is Δt′ (Δt′<Δt) (FIG. 15B), and hence punching is started earlier than proper timing.
To circumvent the situation where punching occurs at a position in front of a target punching position, it is necessary to set a short advance time period so that punching can be started after a sheet is reliably stopped. For example, when the immediately preceding sheet to be punched is a thick sheet and a sheet to be punched this time is a pain sheet, it is necessary to set a short advance time period so as to retard activation of the punch motor 221.
Thus, when the advance time period is set at a predetermined value, the predetermined time period needs to be a uniform value at which activation of the punch motor 221 can be the latest with consideration given to a thickness of each sheet.
The longer the advance time period, the earlier the start of the punch motor 221 and the higher the productivity. According to the prior art, however, even in a case where only plain sheets are punched as a result, punching is carried out with a uniform advance time period set as long as there is a possibility that a thick sheet is punched. For this reason, there is a problem of productivity being not enhanced to a sufficient degree.
Thus, remedial measures are desired so as to deal with the situation where the effect of shortening punching time by setting an advance time period cannot be obtained to a satisfactory level due to the slider overrunning after the punch motor 221 stops.